Resonant versus nonresonant nuclear excitation of 115 In by positron annihilation

Abstract

We have measured the resonant cross section σn for nuclear excitation of 115In via the radiationless annihilation of a positron with a K-shell electron using a monoenergetic positron beam and a thin In target. We find an upper limit on the resonant cross section σn<4.3×10−26cm2 at a 99% confidence level, compared to the cross section σβ=1.7×10−25cm2 determined by two previous measurements of nuclear excitation of 115In using the broad spectrum of positrons from the beta decay of 64Cu. Together these results imply the existence of a hitherto unidentified nonresonant channel for nuclear excitation via energetic positrons.